5.1 is a pain with computer speakers because the wires are usually to short. For most my customers i tell them to get the best 2.1 system they can afford.
As for computer speakers logitech and kilpsch are generally accepted to be the best. I have a set of Logitech Z-5500 that sound good but not nearly as good as my buddies bose system.
The other thing you need to look at is what are you sources and what are you pushing them with. Speakers like the Z-5500's are a waste of money if you pushing them with onboard sound or playing music ripped at 128
Also THX in it's hayday used to be a good standard, but even then it was flawed because it cost companies a small fortune in licensing to get certified, causing many to skip getting certified even though their product meet the standards. Then Lucas sold the THX standard to Creative and it's plummeted since, notice almost all creative products have the THX logo. It's like GM making the h2 and h3 they have nothing to do with the original Humvee, but whoever owns the name can do what they want with it.
Simple answer logitech and kilpsch are the two standards of computer speakers with kilpsch generally being considered the better.